Preparation and Characteristics of Core-Shell Structure with ...practical use of Si is limited due to the huge volume change associated with Si-Li alloying/de-alloying. Here, we report

J. Electrochem. Sci. Technol., 2021, 12(1), 74-81

− 74 −

Preparation and Characteristics of Core-Shell Structure with Nano

Si/Graphite Nanosheets Hybrid Layers Coated on Spherical Natural

Graphite as Anode Material for Lithium-ion Batteries

Hae-Jun Kwon, Jong-In Son, and Sung-Man Lee*

Department of Materials Science & Engineering, Kangwon National University, 1 Gangwondaehakgil, Chuncheon-Si,

Gangwon-Do, 24341, Republic of Korea

ABSTRACT

Silicon (Si) is recognized as a promising anode material for high-energy-density lithium-ion batteries. However, under a

condition of electrode comparable to commercial graphite anodes with low binder content and a high electrode density, the

practical use of Si is limited due to the huge volume change associated with Si-Li alloying/de-alloying. Here, we report a novel

core-shell composite, having a reversible capacity of ~ 500 mAh g-1, by forming a shell composed of a mixture of nano-Si,

graphite nanosheets and a pitch carbon on a spherical natural graphite particle. The electrochemical measurements are per-

formed using electrodes with 2 wt % styrene butadiene rubber (SBR) and 2 wt.% carboxymethyl cellulose (CMC) binder in

an electrode density of ~ 1.6 g cm-3. The core-shell composites having the reversible capacity of 478 mAh g-1 shows the out-

standing capacity retention of 99% after 100 cycles with the initial coulombic efficiency of 90%. The heterostructure of core-

shell composites appears to be very effective in buffering the volume change of Si during cycling.

Keywords : Core-Shell Composites, Si-Graphite Composite Anode, Electrochemical Performance, Anode Material, Lith-

ium-Ion Battery

Received : 13 August 2020, Accepted : 25 August 2020

1. Introduction

The lithium-ion batteries (LiBs) are currently the

most promising energy storage device in a broad

range of applications including electric vehicles as

well as portable electronics. There is an increasing

demand for the higher energy density of LiBs. One of

approaches to meet the ever-growing demand for

increasing the energy density is to substitute the

existing electrode materials with high-capacity mate-

rials [1-4]. Silicon (Si) is considered as one of the

most promising materials due to its high theoretical

specific capacity (3579 mAh g-1 based on the forma-

tion of the Li15Si4 alloy), which is greatly higher than

that of graphite (372 mAh g-1, LiC6). However, Si has

crucial drawbacks for the commercial application in

current LiBs due to the serious volume change

(~300%) during alloying/de-alloying reaction with Li

and its low electrical conductivity, resulting in the

rapid capacity fading during cycling [5,6]. Numer-

ous strategies have been applied to mitigate the

above-mentioned problems for Si-based anodes,

including Si size control, surface coating, active/inac-

tive alloy, void space control and composites, etc.

(refer to recent review paper) [7-11]. Of all the strate-

gies, the approach of carbon/Si composites (CSC)

attracts a great deal of interest because it is believed

to enhance the electrical conductivity and also

accommodate the huge volume changes. It is known,

however, that in practical LiBs, there is little room for

swelling arising from volume expansion of electrodes

[12-14]. The expansion of CSC during charging is

strongly dependent on the fraction of Si in CSC, and

thus the Si content of CSC needs to be low enough to

accommodate the volume change caused by Si, while

resulting in a decrease in capacity of a CSC anode.

In addition, according to previous report [15], it

appears that the specific capacity of LiBs can be sig-

Research Article

*E-mail address: [email protected]

DOI: https://doi.org/10.33961/jecst.2020.01354

This is an open-access article distributed under the terms of the Creative CommonsAttribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0)which permits unrestricted non-commercial use, distribution, and reproduction in anymedium, provided the original work is properly cited.

Hae-Jun Kwon et al. / J. Electrochem. Sci. Technol., 2021, 12(1), 74-81 75

nificantly improved when the current anode material

of graphite is replaced with anode materials having

higher specific capacities compared with conven-

tional graphite anodes.

In this work, we have prepared CSC anode materi-

als having capacity of ~ 500 mAh g-1 as substitute for

the existing graphite and investigated its electro-

chemical performance as an anode material for LiBs.

The core-shell graphite@Si-graphite nanosheet-car-

bon composites were synthesized as a CSC anode

material. In particular, considering the practical stan-

dard, the CSC electrodes for electrochemical evalua-

tion were fabricated with water-based SBR-CMC

binder as in the conventional graphite anode.

2. Experimental

The core-shell graphite@Si-graphite nanosheet-

carbon composites (abbreviated to ‘core-shell com-

posite’) were prepared as follows. A mixture of nano-

Si (99.9%, D50 = ~100 nm, Nanostructured &

Amorphous Materials Inc.), graphite nanosheets

(D50 = 10 µm) and a petroleum pitch (carbon yield,

64%) powders was mixed with a tetrahydrofuran

solution in which the pitch as carbon precursor was

dissolved, and then vacuum-dried at 100oC for 10 h.

The dried mixture was coated on spherical natural

graphite (SNG, POSCO Chemical Co. Ltd., D50 =

16 µm) as the core particle. The process for shell

coating was carried out by using a homemade mixer /

agglomerator. The heat-treatment for carbonization

of pitch was performed under an argon atmosphere at

1000oC.

The particle morphology and cross-section were

examined by scanning electron microscopy (SEM)

with energy dispersive x-ray (EDX) equipment. The

phase information of the core-shell composite pow-

ders was investigated using powder X-ray diffrac-

tometry (XRD) with Cu Kα radiation.

Electrodes for electrochemical measurements were

prepared as follows. Slurries containing 95 wt.%

active material, 1 wt.% carbon black and 2 wt.% sty-

rene butadiene rubber (SBR) and 2 wt.% car-

boxymethyl cellulose (CMC) as a binder, dissolved

in distilled water. The obtained slurries were coated

onto a copper foil that acts as a current collector. The

loading was fixed at ~ 5 mg cm-2. The fabricated

electrodes were dried at 180oC for 12 h under vac-

uum and then pressed.

The electrochemical performance of the prepared

composites was investigated using lithium half-cell

system based on CR2032 coin-type cell. The electro-

lyte was 1M LiPF6 dissolved in a mixed solvent of

ethylene carbonate (EC) and diethyl carbonate

(DEC) (1:1 by volume) with 10 vol % of fluoroeth-

ylene carbonate (FEC) (ENCHEM Co. Ltd, Korea).

The cells were galvanostatically charged (lithiation)

in constant current-constant voltage (CC-CV) and

discharged (de-lithiation) under a constant current

(CC) within the voltage window of 0.01 and 1.0 V at

0.3 C rate at 30oC. To examine the electrode swelling

behavior, the thickness change at different charge-

discharge cycles was also measured in micro-scale

with a micrometer.

3. Results and Discussion

The SEM images of nano-Si and graphite nanosheets,

used for shell coating, and spherical natural graphite

as the core particle are shown in Fig. 1(a), (b) and (c),

respectively. The thickness of graphite nanosheets

was between 20 and 50 nm, as shown in inset Fig.

1(b). The core-shell composite material was prepared

in three different weight ratios as listed in Table 1.

From the SEM images of the core-shell composite

particles and corresponding elemental mappings of

carbon and silicon shown in Fig. 2, the resultant core-

shell composite particles show a spherical morphol-

Fig. 1. Field emission-scanning electron microscopy (FE-SEM) images of (a) nano silicon, (b) graphite nanosheet (inset:

enlarged view of edge), and (c) spherical natural graphite.

76 Hae-Jun Kwon et al. / J. Electrochem. Sci. Technol., 2021, 12(1), 74-81

ogy and the silicon is uniformly distributed amongst

core-shell composite particles. Fig. 3 shows cross-

sectional SEM images of the core-shell composite

material, composite C-II. It is seen that the silicon

particles are distributed between graphite nanosheets

and between graphite nanosheet and graphite core

particle, which are bound using pitch carbon as the

conductive binder as schematically described in Fig.

3c.

Fig. 4 compares the paricle size distributions of the

spherical natural graphite used as core particle and

core-shell composite materials. The distribution of

core-shell composite materials shifts from the core

material to the larger size, indicating that the mixture

of nano-Si and graphite nanosheets are well adhered

to the surface of the core particles.

The XRD patterns of core-shell composite materi-

als are shown in Fig. 5. Only the crystalline diffrac-

tion peaks of the silicon and graphite are observed,

which shows that any impurity phase such as SiC has

not been formed during preparation of core-shell

composite materials.

The charge-discharge curves of core-shell compos-

ite samples during the first and second cycles are

Fig. 2. SEM images of the core-shell composite particles and corresponding elemental mappings of carbon and silicon: (a)

C-I, (b) C-II, (c) C-III.

Table 1. Weight ratio of spherical natural graphite, nano Si, graphite nanosheet, pitch carbon in core-shell composite

materials

Sample nameWeight ratio (wt%)

Spherical natural graphite Nano Si Graphite nanosheet Pitch carbon

C-I 73 9 5 13

C-II 73 8 6 13

C-III 73 7 7 13

Hae-Jun Kwon et al. / J. Electrochem. Sci. Technol., 2021, 12(1), 74-81 77

shown in Fig. 6. During the discharge (delithiation)

process, there is a voltage plateau at around 0.4 V,

corresponding to the de-alloying of Li15Si4 [11,16]. It

appears that the capacity related with the plateau

increases as the silicon content in the composite

materials increases, and the specific capacity of core-

shell composite materials is correspondingly high.

The reversible capacity was 535, 505 and 478 mAh g-1

for C-I, C-II and C-III, respectively. On the other

hand, three composite samples show the similar ini-

tial coulombic efficiency of around 90%.

The cycling performances of C-I, C-II and C-III

electrodes are presented in Fig. 7. In particular, the C-

III electrode exhibits an excellent performance com-

pared with other electrodes, showing 99.2% capacity

retention after 100 cycles (Fig. 7b). In addition, the

coulombic efficiency of C-II and C-III electrodes

quickly increases to 99.5% within the first five cycles

and then reaches 99.8%, while C-I electrode takes 80

cycles to reach above 95% as shown in Fig. 7c. Fig. 8

illustrates the charge-discharge voltage profiles of the

C-I, C-II and C-III electrodes at the 10th, 50th and

100th cycle. In the case of electrodes with a higher Si

content, especially C-I, the voltage plateau at around

0.4 V during discharge gradually decreases in the

subsequent cycles, resulting in a capacity fade during

the cycling. However, the C-III electrode shows a sta-

ble discharge plateau, indicating an excellent cycling

stability. It is also notable that the polarization, as

measured by the voltage drop at the cut-off voltage

for the discharge reaction, increases significantly

with cycling for C-I electrode but remains almost

unchanged from the 10th to the 100th cycle for the C-

III electrode, representing enhanced structural stabil-

ity of the C-III electrode.

It is well known that the degradation of Si-based

electrodes during cycling is attributed to mainly two

mechanisms of the electrical disconnection between

Fig. 3. Cross-sectional SEM image of core-shell composite

material, C-II: (a) low-magnification image and (b) high-

magnification images of selected areas in Fig. 3a and (c)

schematic representation of core-shell composite.

Fig. 4. Particle size distribution analysis of core material

and core-shell composites.

Fig. 5. X-ray diffraction(XRD) analysis of core-shell

composite materials.

78 Hae-Jun Kwon et al. / J. Electrochem. Sci. Technol., 2021, 12(1), 74-81

electrode components, such as active materials and

current collector, and the continuous formation of

solid electrolyte interphase (SEI) layer. Therefore,

the capacity retention of Si-based electrodes during

cycling has been expressed in terms of cumulated rel-

ative irreversible capacities (RIC) defined as the ratio

between the irreversible capacity loss and the deliv-

ered charge capacity [17]. The cumulated loss of

RIC(disconnection) related with electrical disconnec-

tion and RIC(SEI) associated with SEI formation are

demonstrated as a function of cycle for the C-I, C-II

and C-III electrodes (Fig. 9). The irreversibility

related to the SEI formation is comparable to each

other, although it is a little bit larger in the C-I elec-

trode than in the C-II and C-III electrodes (Fig. 9a).

However, there is a distinct difference between those

electrodes concerning the irreversible capacity loss

related with electrical disconnection, as shown in Fig.

Fig. 6. First and second charge-discharge voltage curves of core-shell composite samples: (a) C-I, (b) C-II, (c) C-III.

Fig. 7. Cycling performance of core-shell composite samples: (a) capacity, (b) capacity retention, (c) coulombic efficiency.

Fig. 8. Charge-discharge voltage profiles at the 10th, 50th, 100th cycle: (a) C-I, (b) C-II, (c) C-III.

Hae-Jun Kwon et al. / J. Electrochem. Sci. Technol., 2021, 12(1), 74-81 79

9b. The irreversible capacity loss is getting higher

with cycling in the C-I electrode, while there is no

noticeable increase in the C-III electrode. It therefore

appears that the capacity fading during cycling of the C-

I electrode is mainly due to an irreversible capacity loss

caused by electrical disconnection. This is consistent

with the results described above, e.g., the polarization

increase and 0.4 V plateau capacity decrease during

cycling observed in the C-I electrode.

The electrode thickness change after 50 cycles was

illustrated for both lithiated and de-lithiated states

(Fig. 10a). Considering that the degree of expansion

increases with the reversible capacity, or the Si con-

tent in the composite, the electrode thickness change

was also normalized by the reversible capacity mea-

sured at the first cycle (Fig. 10b), which shows simi-

lar behavior to Fig. 10a. As expected, the C-III

electrode exhibits the lowest expansion rate of 23%

in the de-lithiated state and 42% in the lithiated state.

It is worth noting here that the gap between the elec-

trode thickness changes after charge and discharge

reaction appears to be 9, 12 and 19% for the C-I, C-II

and C-III electrodes, respectively. In general, when

an electrode is discharged, lithium ions could not be

extracted from active materials isolated through elec-

trical disconnection, resulting in still swollen state

even after discharge process. Therefore, the C-I elec-

trode, having a relatively high degree of the irrevers-

ibility related with electrical disconnection as shown

in Fig. 9b, reveals a relatively small difference in

Fig. 9. Relative irreversible capacities (RIC) analysis of core-shell composite materials associated with (a) SEI formation,

and (b) electrical disconnection.

Fig. 10. Electrode thickness change of core-shell composite materials for lithiated and delithiated states after 50th cycle: (a)

measured thickness change, (b) thickness change normalized by the reversible capacity.

80 Hae-Jun Kwon et al. / J. Electrochem. Sci. Technol., 2021, 12(1), 74-81

electrode expansion rate between charged and dis-

charged states. In contrast, the C-III electrode shows

its mechanical resilience and can accommodate the

volume change of electrode during cycling.

This relatively poor performance of the C-I elec-

trode, containing a higher Si content than the other

electrodes, could be attributed the following reasons:

(i) some nano-Si particles are not wrapped in graphite

nanosheets and exposed to electrolyte and (ii) some

nano-Si particles are aggregated and electrically dis-

connected during cycling. On the other hand, the

well-designed core-shell composite, in which the sili-

con particles are uniformly distributed in a pitch car-

bon matrix between graphite nanosheets and between

graphite nanosheet / graphite core particle, can result

in the excellent electrochemical performance as in

the C-III electrode. It should be noted here that the

electrochemical measurements were performed using

electrodes with 4 wt% SBR-CMC binder in a high

electrode density (~ 1.6 g cm-3), which is comparable

to commercial graphite anodes. Fig. 11 shows the

surface morphology of the C-I, C-II and C-III elec-

trodes before and after 50 cycles. After 50 cycles, no

cracks are formed in all the electrodes, and, espe-

cially in the C-III electrode, there is no signature of

mechanical damage in the composite particles. At

this point, it would be important to mention the effect

of the graphite nanosheet on the electrode swelling.

The electrode thickness change during cycling of a

core-shell composite, in which the shell is composed of

a mixture of nano-Si and a pitch carbon without graphite

sheets, is compared with that of the C-II and C-III com-

posites (Fig. 12). It should be noted here that the revers-

ible capacity of the core-shell composite (490 mAh g-1)

was comparable with that of the C-II and C-III compos-

ites. The electrode thickness change was measured at

the lithiated state. Both C-II and C-III electrodes exhib-

ited milder electrode expansion trends than the core-

shell composite with a shell not containing graphite

nanosheets.

Given that the difference is the presence of graphite

nanosheets in shell, the less significant electrode expan-

sion in C-II and C-III electrodes can be attributed to the

buffering role of graphite nanosheets in shell of the core-

shell composite. These results indicate that the hetero-

structure of core-shell composite, prepared by forming a

shell composed of a mixture of nano-Si, graphite

nanosheets and a pitch carbon on a spherical natural

graphite particle, is very effective in buffering the vol-

ume change of Si during cycling.

4. Conclusions

In summary, we designed and fabricated a novel

Fig. 11. Surface morphology of the core-shell composite

electrodes before and after 50 cycles: (a,b) C-I, (c,d) C-II

and (e,f) C-III.

Fig. 12. Change in the electrode thickness at the lithiated

state over 50 cycles of the three composites : a core-shell

composite with a shell not containing graphite nanosheets,

C-I and C-III.

Hae-Jun Kwon et al. / J. Electrochem. Sci. Technol., 2021, 12(1), 74-81 81

core-shell composite, having a reversible capacity of

~ 500 mAh g-1, by forming a shell composed of a

mixture of nano-Si, graphite nanosheets and a pitch

carbon on a spherical natural graphite particle. Due to

the unique structure of nano-Si effectively wrapped

by highly conduct ive and f lexible graphite

nanosheets in a shell, nano-Si particles are allowed to

expand freely without mechanical constrain during

lithiation and thus the core-shell composites can

accommodate the volume change of electrode during

charge and discharge. As a result, we have achieved a

reversible capacity of ~ 500 mAh g-1 with the initial

coulombic efficiency of 90%. The core-shell com-

posites show the outstanding cycling stability in an

electrode prepared in an electrode density of ~ 1.6 g

cm-3 with 4 wt% SBR-CMC binder. The heterostruc-

ture of core-shell composites appears to be very

effective in buffering the volume change of Si during

cycling.

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